Inert gas stripping of contaminated water

ABSTRACT

Water containing contaminating gases such as hydrogen sulfide, carbon dioxide and ammonia is freed of these contaminants by closed system stripping process which employs an inert gas and steam.

@nfieed saws 9999M 1191 Kent 1 51 Aug. 28, 1973 INERT GAS STRIPPING OF CONTAMINATED WATER [75] Inventor: Robert D. Kent, Bellaire, Tex.

[73] Assignee: Texaco Inc., New York, N.Y.

{22] Filed: Feb. 1, 1972 211 Appl. 190.; 222,430

Primary Examiner-Samih N. Zaharna Assistant Examiner-Richard W. Burks Attorney-Thomas H. Whaley et a1.

55/73 57 ABSTRACT [51] Int. Cl. 80141 19/00, 801121 53/14 1 [58] Field of Search 55/46, 47, 48, 51-54, water containing cumaminafing gases such as hydm 55/70' 198 gen sulfide, carbon dioxide and ammonia is freed of these contaminants by closed system stripping process [56] Rderenc which employs an inert gas and steam.

UNITED STATES PATENTS 2,141,349 12/1938 Engelhardt 55 51 9 Claims, 2 Drawing Figures CLOSED CYCLE INERT GAS STRIPPING SINGLE ABSORBER FOR AMMONIA AND HYDROGEN SULF/DE ggggpvgrE I AM/NE ABSORBER 12 PS/G 7 n J {00F 3PSIG v? REFLUX LEAN AMINE V ACCUMULATOR DRUM 15 STEAM 1 9 V, 3 Lil 1a PS/G T RICH AM/NE STRIPPED M g g f ZPSIG x Q 100 V 2370 7 r --COMPRESSOR FEED BOTTOMS INERT GAS RECYCLE 2 EXCHANGER sOuR WATER FEED INERT GAS STRIPPING OF CONTAMINATED WATER BACKGROUND OF THE INVENTION This invention concerns the field of separation processes. Specifically it concerns the field of separating contaminating gases from water. More specifically it concerns the use of a stripper to remove contaminating gases from water and one or more absorbers to recover these contaminants.

The effluent water from refinery and chemical pro cesses often contains dissolved gases such as hydrogen sulfide, ammonia and carbon dioxide. These materials are pollutants and must be removed from water before it is discharged or is suitable for reuse. Until recently such pollutants have generally been removed by stripping with steam or less frequently with flue gas and the resultant overhead discharged to a fired heater or other burning equipment whereby the pollutants are incinerated and discharged from stacks. Lately, laws have been enacted in many areas whereby the amount of pollutants that may be so incinerated and discharged from stacks is strictly limited, generally requiring disposal of said pollutants by other means. One means that has recently come into use is to strip the water with steam as before but pass the stripper overhead gases containing the pollutants through one or more absorbers whereby the pollutants are absorbed for subsequent processing or disposal. In this manner the incineration of the pollutants is avoided and furthermore they can often be recovered as valuable byproducts. Flue gas cannot generally be employed because of reactive materials present such as carbon dioxide, oxygen, sulfur dioxide and the like.

Prior to my invention steam alone was generally employed to strip these constituents from water. Steam stripping, however, has several disadvantages. Steam stripping is only satisfactory as long as the temperature of the stripper overhead can be maintained at a high enough level to carry overhead a substantial percentage of water vapor (generally 35 percent by volume or more) along with the hydrogen sulfide, ammonia and carbon dioxide. However, it is often necessary to reduce the overhead temperature due to processing requirements of the stripper overhead gas. As the overhead temperature drops the water vapor content of the stripper overhead becomes less and the overhead becomes highly concentrated in hydrogen sulfide, ammonia and carbon dioxide. This results in top tray accumulation of hydrogen sulfide, ammonia bisulfide and ammonium carbonate which makes separation to an ideal bottoms product in the order of one part per million (PPM) hydrogen sulfide difficult and substantially increases corrosion of the top tray. Also, plugging is likely to occur in the overhead lines and equipment due to deposition of ammonium salts (bisulfide, carbamate, bicarbonate, and carbonate). Also, since steam stripping involves the carryover of large quantities of water with the stripped gases a problem may occur downstream of the stripping tower with the absorbers which must dissolve the stripped gases. These absorbers are typically solvent systems which preferentially dissolve a given contaminant. If water is present it too may be dissolved to some extent in the solvents. Since these solvents are expensive and must be recycled, any water they pick up must be removed. This process of removing the water from the absorber solvents becomes increasingly difficult and more expensive if large amounts of water are dissolved in the solvents as is the case with steam stripping systems which must necessarily have large amounts of water in contact with the absorbers. These difficulties may be overcome by the process of my invention which briefly involves stripping contaminants from water using an inert gas mixed with steam in a closed system where the inert gas is recovered and reused by recycling with use of a compressor.

Use of an inert gas mixed with steam has an advantage over steam alone as a stripping medium because a larger partial pressure of inert gas relative to the partial pressure of ammonia, carbon dioxide and hydrogen sulfide may be obtained at a given temperature using some inert gas than would the partial pressure of steam alone relative to the partial pressure of ammonia, carbon dioxide and hydrogen sulfide at a given temperature. Thus, at a given temperature ammonia, carbon dioxide and hydrogen sulfide will be more efficiently stripped if some inert gas is present as a stripping medium. Also, solids will have less tendency to form when some inert gas is used as a stripping medium since the equilibrium between solids such as ammonium bisulfide and ammonium carbonate, for instance, will be more in favor of the gases making up the solids.

SUMMARY OF THE INVENTION My invention is a process wherein dissolved gaseous contaminants are stripped from water containing dissolved amounts of these gases by an inert gas and steam mixture circulating in a closed system such that the mixture of inert gas and stripped gases is passed through absorbing means to separate the inert gas from the stripped gases. The inert gas is then recycled by means of a compressor to be reused for stripping purposes.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 depicts a process of my invention in which one absorber is required to remove the contaminants from the stripping tower overhead stream. FIG. 2 depicts a process of my invention in which a plurality of absorbers is required to remove the contaminants from the stripping tower overhead stream.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The process of my invention may be better understood by reference to the attached drawings.

In a typical embodiment of my invention as shown in FIG. I, water 10 containing contaminants including hydrogen sulfide, ammonia and carbon dioxide is introduced near the top tray of a conventional stripping tower l l. Inert gas enters the stripping tower at or near the bottom at l2. Steam is also introduced at or near the bottom of the stripping tower at 13. Alternately, heat may be applied to the bottoms by use of a reboiler or the like to generate steam from the bottoms liquid. The water stripped of contaminants leaves the bottom of the stripping tower at 14. The inert gas, steam, and stripped contaminants are routed through a condenser and reflux system 15 to increase the efficiency of the stripping tower. The inert gas and stripped contaminants (including hydrogen sulfide, ammonia and carbon dioxide) leave the stripping tower and the reflux accumulator drum and are passed through an absorber 16 wherein the hydrogen sulfide and ammonia are removed by a suitable solvent. The inert gas now relatively free of contaminants leaves the absorber at 17 and is reintroduced by means of a compressor to the original stripping tower at 12 and the process is rement of high temperature alleviates the problems of corrosion and solids formation which occur when steam alone is used as a stripping medium.

After the stripped contaminants are removed from peated. 5 the water by the steam and inert gas they are passed Another embodiment of the process of my invention through absorber means where the contaminants are is shown in FIG. 2. Water containing contaminants preferentially removed from the inert gas usually by including hydrogen sulfide, ammonia and carbon dioxdissolution in a solvent. Some systems use separate reide is introduced near the top tray of a conventional covery systems for each contaminant while others use stripping tower ll. Inert gas enters the stripping tower 10 a single unit to recover all shipped contaminants. Typinear the bottom at 12. Steam is also introduced near cal solvents are amines such as mono-ethanol amine the bottom of the stripping tower at 13. Altemately, and diethanol amine, for example. The methods of abheat may be applied to the bottoms by use of a reboiler sorption of gases such as ammonia and hydrogen sulor the like to generate steam from the bottoms liquid. fide are well known to those skilled in the art. Also, The water stripped of contaminants leaves the bottom since the process of my invention has a reflux drum of the stripping tower at 14. The inert gas, steam and temperature of about 100 F. compared with 180 F. stripped contaminants are routed through a condenser for steam alone there is better absorption of the conand reflux system 15 to increase the efficiency of the taminating gases by the absorbers downstream and less stripping tower. The inert gas and stripped contamiwater is available to contaminate the absorber solvents. nants (including hydrogen sulfide, ammonia and car- After the absorbers have removed the other gases the bon dioxide) leave the stripping tower and its reflux acinert gas is returned to the stripping tower by means of cumulator drum and are passed through an absorber 16 a compressor to be reused in removing carbon dioxide, wherein a solvent removes the ammonia. The remainammonia and hydrogen sulfide from water. Thus, the ing gases pass through another absorber 17 wherein anhigh utility cost for steam stripping is reduced. A lower other solvent removes the hydrogen sulfide. The inert utility cost for power to drive the compressor is substigas now relatively free of contaminants leaves the hytuted. On the basis of 50 cents/1,000 lb. for steam, and drogen sulfide absorber at 18 and is reintroduced to the l cent/KWI-I for power, utility costs are reduced as original stripping tower at 12. inert gas is substituted for steam. For each 1,000 lbs/hr.

The inert gas used in my invention must be unreacof steam that is replaced with inert gas, utility cost is retive with any of the contaminating constituents in the duced by 30 cents/hr. Even if the inert gas is slightly water or the solvents used in the absorbers. Inert gas is leSS effici nt than Steam the Cost avings w uld be subexpensive relative to steam and has not been widel stantial. The reason for possible less efficiency of the used for stripping purposes heretofore dispite its operainert gas lies in the fact that the inert gas being recycled tional advantages. My invention avoids the economic to the stripping tower may obtain minute amounts of problems of inert gas by recycling and reusing the inert contaminants depending on the efficiency of the abgas by use of a compressor. sorbers. In order that the water leaving the bottom of The inert gases suitable for use in the process of my the tower may be completely free of contaminants the invention include, for example, nitrogen and hydrogen. recycled inert gas should be introduced a few trays up In certain cases carbon dioxide would be inert and from he bottom of the Stripping tower. Pure inert gas could be used. An especially preferred gas is nitrogen. Cou d be t oduce at the bottom of the stripping The process of my invention does not ordinarily use tower and no e e cy loss ould occur. inert gas alone. Some steam is generally required in ad- In add o s ripping S m Some additional heatdition to the inert gas as a stripping medium. The may be needed, from any convenient source, to heat amount of steam relative to the total of steam and inert the contaminants water feed up to the bottoms tempergas may range from 10 to 85 mole percent. Table A ature of the stripping tower. For a stripper operating at shows conditions for stripping tower operation using 20 PSIG at the bottom, stripper bottoms temperatures various mixtures of steam and nitrogen as stripping mewould range from about 260 F. with very little inert diums. The use of steam alone is also shown for comgas being used to about 150 F. with a large percentage parative purposes. From the table it may be noted that of inert gas being used. Equivalent feed temperature the reflux drum temperature is lower when using inert for the contaminated water would then be about 225 gas than when steam alone is used. The lack of require- F. to about 120 F.

TABLE A Calculated Conditions for Mixtures of Inert Gas and Steam for Sour Water Stripper Inert gas and steam Steam only STRIPPING MEDIUM Nitrogen. mph* 300 600 900 1200 1500 1800 1980 0 Steam, mph 1900 I600 1300 1000 700 400 220 1200 Total 2200 2200 2200 2200 2200 2200 "200 2200 Mol percent steam 86.5 72.8 59.1 45.5 31.8 18.2 10.0 100.0 Partial pressure steam.. 28.5 24.0 19.5 14.8 10.5 6.0 3.3 33.0 Bottom temp.. "F 247 238 226 213 I 170 I45 256 Reflux drum temp. "F 100 100 100 100 100 I00 I80 OVERHEAD PRODUCT W tDRY BASIS) Ammonia, mph .V.. 82 82 82 82 82 82 82 82 Hydrogen ulfide, mplfi 68 68 68 68 68 68 68 78 inert gas and steam St am only Carbon 516x55. in h 4 4 4 4 4 4 4 4 Nitrogen. mph" 300 600 900 1200 1500 1800 1980 Total 454 754 1054 1354 1654 1954 2134 1:4

Mol percent water in vapor. 3.52 3.52 3.52 3.52 3.52 3.52 3.52 27.8 Mph* water 16.5 27.5 38.2 49.0 58.0 69-0 78 5.9

Ttoal 470 781 1092 1403 1714 025 2 3 13 MOL PERCENT OVERHEAD PRODUCT COMPOSITION Ammonia 17.5 10.5 7.5 5.8 4.8 4.11 3.7 38.5 Hydrogen sulfide. 14.5 8.7 6.2 4.8 3.9 3.4 3.0 31.8 Carbon dixode. 0.8 0.5 0.4 0.3 0.2 0.2 0.2 1.9 Nitrogen 63.7 76.8 82.4 85.6 90.6 as 89.6 11 Water 3.5 3.5 3.5 3.5 3.5 3.5 3.5 27 it Total 100.0 100.11 100.0 1110.1) 1110.0 11111.0 100.11 1110.0

Mules per hour.

I claim: taminants from the water with steam and inert gas, said I. A process for removing gaseous contaminants inert gas flowing in a closed path wherein the inert gas from water which comprises: is continuously reused for stripping.

stripping the contaminants from the water with a 6. The process of claim 5 wherein the amount of mixture of an inert gas and steam and, steam relative to the total of steam and inert gas is from recovering the inert gas and repeating the stripping 30 about 10 mole percent to about 85 mole percent.

process continuously. 7. The process of claim 6 wherein the inert gas is ni- 2. A process as in claim 1 wherein the amount of trogen. steam relative to the total of steam and inert gas is from 8. The process of claim 5 wherein the contaminants about 10 mole percent to about 85 mole percent. include hydrogen sulfide and ammonia.

3. The process of claim 1 wherein the inert gas is ni- 9. The process of claim 5 wherein the inert gas is trogen. freed of the contaminants picked up in the stripping 4. The process of claim 1 wherein the water contamiprocess by contacting the inert gas and contaminants nants include hydrogen sulfide and ammonia. with a solvent system which will selectively dissolve 5. A process for removing gaseous contaminants only the contaminants. from water which comprlses stripping the gaseous con- 

2. A process as in claim 1 wherein the amount of steam relative to the total of steam and inert gas is from about 10 mole percent to about 85 mole percent.
 3. The process of claim 1 wherein the inert gas is nitrogen.
 4. The process of claim 1 wherein the water contaminants include hydrogen sulfide and ammonia.
 5. A process for removing gaseous contaminants from water which comprises stripping the gaseous contaminants from the water with steam and inert gas, said inert gas flowing in a closed path wherein the inert gas is continuously reused for stripping.
 6. The process of claim 5 wherein the amount of steam relative to the total of steam and inert gas is from about 10 mole percent to about 85 mole percent.
 7. The process of claim 6 wherein the inert gas is nitrogen.
 8. The process of claim 5 wherein the contaminants include hydrogen sulfide and ammonia.
 9. The process of claim 5 wherein the inert gas is freed of the contaminants picked up in the stripping process by contacting the inert gas and contaminants with a solvent system which will selectively dissolve only the contaminants. 